Cercopagis pengoi is the latest exotic crustacean to invade the Great Lakes. This predatory cladoceran was first identified by Canadian scientists in early August of 1998, and was reported via Internet by Dr. Hugh MacIsaac at the end of August. Cercopagis is indigenous to the Caspian, Azov, and Aral seas (Rivier 1998), and was reported to have invaded the Baltic Sea in 1992 (Ojaveer & Lumberg 1995, Ojaveer 1997).

Cercopagis is similar to another recent invader in the Great Lakes, Bythotrephes cederstroemi. Both Cercopagis and Bythotrephes belong to the family Cercopagididae, and have long caudal processes with up to three pairs of barbs along the proximal end of the process. Both species occur in brackish and pure freshwater environments. In addition to sexual reproduction, Cercopagidids most commonly reproduce parthenogenically, which allows them to quickly establish new populations with a relatively small seed population.

The Great Lakes National Program Office monitors biological and chemical data across all five Laurentian Great Lakes during two annual surveys, one in spring and one in summer. Currently the zooplankton program takes vertical tows from depths of 20 and 100 meters, using 63 and 153 micron mesh nets, respectively. In 1998 the summer survey included a total of 72 sites, with 8 sites sampled in Lake Ontario between August 5-7. Cercopagis was found in 4 of these 8 sites, all occurring in the central basin Lake Erie was sampled August 2-4, and no Cercopagis were found at any of the 20 sites sampled.

Densities in Figure 1 were calculated from the 20 meter tow, since it is believed that Cercopagis generally reside in the warmer, upper waters. However, because of their large eye and brood sac, they are highly vulnerable to predation by larger planktivorous fishes (Ojaveer & Lumberg 1995). To avoid predation and possibly to follow migrating prey, Cercopagis do migrate below 20 meters during the day, as shown in Figure 2 below. In the early afternoon (site 49), high densities of Cercopagis were found below 20 meters. In contrast, all Cercopagis were above 20 meters at sunrise (site 55).

Body length of Cercopagis from site 55 was measured to the nearest 0.1mm. Only animals with all 3 barb pairs present were measured, and when the entire caudal process was present, the full length of the animal was measured. Figure 3 presents body lengths (including caudal process) of the three categories of individuals encountered: sexual females with resting eggs, parthenogenic females with embryos in the brood sac, and males, and Figure 4 presents body lengths without the caudal process. The gray shaded boxes are the ranges for Cercapagis from the Caspian-Azov Sea area (Rivier 1998). Both the females and males from Lake Ontario exceed the size range of those found in the Caspian-Azov Sea.

Figure 3: Body Length of Cercopagis pengoi (including caudal process)

It is unknown at this point how long Cercopagis has inhabited the Great Lakes before first being reported, and what future impacts it may have. Given the linkages between Lake Ontario and the other lakes, it is likely that this animal will spread throughout the lakes in time. Given the high densities observed during the summer 1998 cruise, it is possible that predation pressure by Cercopagis on smaller cladocerans can affect both the size and composition of phytoplankton communities. In addition, Cercopagis may impact fish populations by competing with 0-year fishes for small prey items, or conversely by becoming prey itself for fishes beyond the 0-year stage.